Low-temperature Scanning Tunneling Spectroscopy Research Articles

Mapping the Dirac point in gated bilayer graphene

We have performed low temperature scanning tunneling spectroscopy measurements on exfoliated bilayer graphene on SiO2. By varying the back gate voltage we observed a linear shift of the Dirac point and an opening of a bandgap due to the perpendicular electric field. In addition to observing a shift in the Dirac point, we also measured its spatial dependence using spatially resolved scanning tunneling spectroscopy. The spatial variation of the Dirac point was not correlated with topographic features and therefore we attribute its shift to random charged impurities.

Kondo-effect of substitutional cobalt impurities at copper surfaces

The influence of the coordination on the Kondo temperature of a magnetic impurity at a noble metal surface and the line shape observed in low temperature scanning tunneling spectroscopy (STS) is investigated for single cobalt atoms adsorbed on and embedded in copper surfaces. Surprisingly, the Kondo temperature for substitutional cobalt atoms is almost the same as that of adatoms on the Cu(100) surface. This is in stark contrast to the behaviour observed at the Cu(111) surface. DFT calculations reveal that in the case of Cu(100) the coupling of the spin of the cobalt atom to the conduction band is not substantially increased by the incorporation of the cobalt atom. At the same time the observed line shape differs strongly from what is observed on adatom systems.

Low-Temperature Scanning Tunneling Microscopy and Spectroscopy Investigation of the Electronic Surface State of Self-Organized Cr Islands on Au(111)

We have investigated the morphologic and electronic properties of self-organized nanosized Cr islands grown on atomically flat Au(111) surfaces by means of low-temperature scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. We observe the existence of an electronic surface state on the Cr islands, which manifests itself through the formation of standing wave patterns within the interior of the atomically flat islands that gain in complexity for increasing bias voltages. The patterns are only weakly observable when compared to similar standing waves formed within self-organized Co islands on Au(111). This difference is attributed to the more irregular shape of the Cr islands. Furthermore, we have found that the presence of the Cr surface state is reflected by the appearance of a distinct electronic state in the -200 mV to -150 mV range, resembling to previous findings for self-organized Co islands on Au(111).

ZnO(0001) surfaces probed by scanning tunneling spectroscopy: Evidence for an inhomogeneous electronic structure

The stability of the polar Zn-terminated ZnO surface is probed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Surface states in the bandgap of ZnO are evidenced by STS and their presence is correlated with the local surface corrugation. Very defective surface regions are characterized by a bulk electronic structure showing a wide bandgap while nanometer-scale defect free regions exhibit a narrower bandgap and surface states. We also image atomically resolved (3×3)R30° reconstructions on the defect-free areas.

Decay mechanisms of excited electrons in quantum-well states of ultrathin Pb islands grown on Si(111): Scanning tunneling spectroscopy and theory

Using low-temperature scanning tunneling spectroscopy at 5 and 50 K, we studied the linewidth of unoccupied quantum-well states in ultrathin Pb islands, grown on Si(111) on two different Pb/Si interfaces. A quantitative analysis of the differential conductance spectra allowed us to determine the electron-electron $(e\text{\ensuremath{-}}e)$, electron-phonon $(e\text{-ph})$ and the interface and defect contributions to the lifetime. Layer-dependent ab initio calculations of the $e\text{-ph}$ linewidth contribution are in excellent agreement with the data. Importantly, the sum of the calculated $e\text{\ensuremath{-}}e$ and $e\text{-ph}$ lifetime broadening follows the experimentally observed quadratic energy dependence.

Identifying Reactive Intermediates in the Ullmann Coupling Reaction by Scanning Tunneling Microscopy and Spectroscopy

We present an atomic-scale study of substituent effects in the Ullmann coupling reaction on Cu{111} using low-temperature scanning tunneling microscopy and spectroscopy. We have observed fluorophenyl intermediates and phenyl intermediates as well as biphenyl products on Cu{111} after exposure to 4-fluoro-1-bromobenzene (p-FC(6)H(4)Br) and bromobenzene (C(6)H(5)Br), respectively. When p-FC(6)H(4)Br dissociatively chemisorbs at 298 K on Cu{111}, the relatively weakly bound Br dissociates, and fluorophenyl intermediates are formed. These intermediates couple to form 4,4'-difluorobiphenyl and desorb at temperatures below 370 K. However, by cooling the substrate to low temperature (4 K), we have observed unreacted fluorophenyl intermediates distributed randomly on terraces and at step edges of the Cu{111} surface. Alternatively, at similar coverages of C(6)H(5)Br, we have observed biphenyl distributed on terraces and step edges. In each case, Br adatoms were randomly distributed on the surface. Chemical identification of fluorophenyl and phenyl intermediates and biphenyl products was achieved by vibrational spectroscopy via inelastic tunneling spectroscopy. The strongest vibrational mode in the phenyl species disappears when the tilted intermediates couple to form biphenyl products. We infer that the surface normal component of the dipole moment is important in determining the transition strength in inelastic electron tunneling spectroscopy.

Reduction of the Superconducting Gap of Ultrathin Pb Islands Grown on Si(111)

The energy gap Delta of superconducting Pb islands grown on Si(111) was probed in situ between 5 and 60 monolayers by low-temperature scanning tunneling spectroscopy. Delta was found to decrease from its bulk value as a function of inverse island thickness. Corresponding T_{c} values, estimated using bulk gap-to-T_{c} ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for freestanding Pb films show that the electron-phonon coupling constant, determining T_{c}, decreases with diminishing film thickness.

Scanning tunneling microscopy and spectroscopy of graphene layers on graphite

We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate.

Modifying the electronic structure of semiconducting single-walled carbon nanotubes byAr+ion irradiation

Local controllable modification of the electronic structure of carbon nanomaterials is important for the development of carbon-based nanoelectronics. By combining density-functional theory simulations with Ar-ion-irradiation experiments and low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) characterization of the irradiated samples, we study the changes in the electronic structure of single-walled carbon nanotubes due to the impacts of energetic ions. As nearly all irradiation-induced defects look as nondistinctive hillocklike features in the STM images, we compare the experimentally measured STS spectra to the computed local density of states of the most typical defects with an aim to identify the type of defects and assess their abundance and effects on the local electronic structure. We show that individual irradiation-induced defects can give rise to single and multiple peaks in the band gap of the semiconducting nanotubes and that a similar effect can be achieved when several defects are close to each other. We further study the stability of defects and their evolution during STM measurements. Our results not only shed light on the abundance of the irradiation-induced defects in carbon nanotubes and their signatures in STS spectra but also suggest a way the STM can be used for engineering the local electronic structure of defected carbon nanotubes.

Spin structure of an atomic protrusion: Probing single atoms on cobalt nanoislands

Single Ni, Co, and Cu atoms deposited in the center of cobalt nanoislands grown on Cu(111) are investigated by low-temperature scanning tunneling spectroscopy. The surface states of this model magnetic nanolead are unveiled and assigned to arise from atomiclike and surface-induced states by ab initio calculations. Contrary to the first, the second contribution is predicted by calculations to favor a change in sign of the spin polarization with respect to the pristine lead.

Superconducting density of states at the border of an amorphous thin film grown by focused-ion-beam

We present very low temperature Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements of a W based amorphous thin film grown with focused-ion-beam. In particular, we address the superconducting properties close to the border, where the thickness of the superconducting film decreases, and the Au substrate emerges. When approaching the Au substrate, the superconducting tunneling conductance strongly increases around the Fermi level, and the quasiparticle peaks do not significantly change its position. Under magnetic fields, the vortex lattice is observed, with vortices positioned very close to the Au substrate.

Dynamic Jahn-Teller effect in electronic transport through singleC60molecules

Low-temperature scanning tunneling spectroscopy and first-principles calculations are used to characterize electron transport through vibronic states of ${\text{C}}_{60}$ molecules. This is achieved by positioning a ${\text{C}}_{60}$ molecule on top of a molecular self-assembled template on Au(111). In these conditions, conductance spectra are shown to reveal the dynamic Jahn-Teller effect of the ${\text{C}}_{60}$ molecule. This vibronic transport study helps in solving a long-standing debate [Phys. Rev. Lett. 74, 1875 (1995); Phys. Rev. Lett. 91, 196402 (2003)] on density-functional calculations of the ${\text{C}}_{60}$ electron-phonon coupling strength.

Electron Damage to Supported Ice Investigated by Scanning Tunneling Microscopy and Spectroscopy

We study the interaction of low-energy electrons with crystalline ice (D2O) on Cu(111) by low-temperature scanning tunneling microscopy and spectroscopy. Electrons induce dissociation of the molecules with a threshold of approximately 3 eV. The large dissociation yield of the order of 10(-8)/electron and the extended area of dissociation are attributed to a shift in conduction band during the dissociation. Voltage dependent differences in imaging of ice and dissociated ice are reflected in the spectroscopic signature.

Metallic Nature of theα−Sn/Ge(111)Surface down to 2.5 K

Low temperature (down to 2.5 K) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are presented to assess the nature of the alpha-Sn/Ge(111) surface. Bias-dependent STM and STS measurements have been used to demonstrate that such a surface preserves a metallic 3 x 3 reconstruction at very low temperature. A tip-surface interaction mechanism becomes active below about 20 K at the alpha-Sn/Ge(111) surface, resulting in an apparent unbuckled (sqrt[3] x sqrt[3]) reconstruction when filled states STM images are acquired with tunneling currents higher than 0.2 nA.

Superconducting Pb Island Nanostructures Studied by Scanning Tunneling Microscopy and Spectroscopy

Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.

Nanoscale superconducting properties of amorphous W-based deposits grown with a focused-ion-beam

We present very low temperature scanning tunneling microscopy and spectroscopy (STM/S) measurements in W-based amorphous superconducting nanodeposits grown using a metal–organic precursor and a focused-ion-beam. The superconducting gap closely follows s-wave Bardeen–Cooper– Schrieffer theory, and STS images under magnetic fields show a hexagonal vortex lattice whose orientation is related to features observed in the topography through STM. Our results demonstrate that the superconducting properties at the surface of these deposits are very homogeneous, down to atomic scale.

Emergence and Visualization of an Interface State during Contact Formation with a Single Molecule

Contact formation dynamics and electronic perturbations arising from the interaction of a metallic probe and a single molecule (1,3 cyclohexadiene) bound on the Si (100) surface are examined using a series of plane wave, density functional theory calculations. The approach of the probe induces a relaxation of the molecule that ultimately leads to the formation of an interface state due to a specific interaction between the probe apex atom and the C=C bond of the molecule. The calculated interface state is located 0.2 eV above the Fermi energy, in agreement with low temperature scanning tunneling spectroscopy local density of states data (0.35 eV), and is responsible for the contrast observed in low bias empty-state STM images.

Morphology and electron confinement properties of Co clusters deposited on Au(111)

We present the first measurements of both the morphology and electronic structure of preformed Co nanoclusters in the size range of a few up to several hundreds of atoms by means of low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Co clusters are produced in the gas phase and deposited under controlled ultra-high vacuum conditions onto clean Au(111) with low density, well below complete coverage of the Au(111) substrate. We find that smaller clusters, which typically contain less than 20 atoms, exhibit a significant surface mobility with subsequent aggregation, whereas larger clusters turn out to be immobile. From a systematic analysis of the cluster height distribution, we infer that the approximately spherical clusters experience only a restricted flattening and have a multilayered structure, which sometimes is observed to exhibit hexagonal facets, pointing to a truncated octahedral shape. Furthermore, detailed STS measurements on individual Co clusters reveal the presence of various size- and shape-dependent maxima with large energy spacings of the order of 100 meV, as well as an occupied state around −200 meV that originates from the Co 3d band. Our findings provide direct evidence for the existence of strong electron confinement effects in the Co clusters stemming from delocalized Co valence electrons. Our results provide an original contribution to the understanding of cluster physics and in particular of the specific electronic structure.

Quantum modulation of the Kondo resonance of Co adatoms on Cu/Co/Cu(100): Low-temperature scanning tunneling spectroscopy study

Low-temperature scanning tunneling spectroscopy reveals that the Kondo temperature ${T}_{K}$ of Co atoms adsorbed on Cu/Co/Cu(100) multilayers varies between 60 and 134 K as the Cu film thickness decreases from 20 to 5 atomic layers. The observed change of ${T}_{K}$ is attributed to a variation of the density of states at the Fermi level ${\ensuremath{\rho}}_{F}$ induced by quantum well states confined to the Cu film. A model calculation based on the quantum oscillations of ${\ensuremath{\rho}}_{F}$ at the belly and the neck of the Cu Fermi surface reproduces most of the features in the measured variation of ${T}_{K}$.

Electronic decoupling and templating of Co nanocluster arrays on the boron nitride nanomesh

Positioning nanostructures and functional molecules at surfaces and engineering their local coupling behavior is a key challenge in nanotechnology. The recently discovered BN nanomesh on the Rh(1 1 1) surface is a potential candidate to combine both properties, templating on the nanometer-scale and electronic decoupling. By means of low temperature scanning tunneling microscopy and spectroscopy experiments we demonstrate this advantageous combination for the case of Co nanoclusters, which can be ordered in a hexagonal nanoarray with a nearest neighbor distance of 3.2 nm on the BN nanomesh. The detection of a Coulomb gap of ≈160 meV proves the electronic decoupling of the clusters from the rhodium substrate.